Infections by intracellular pathogens are a major cause of neonatal morbidity and mortality. This proposal seeks to characterize aspects of the neonates' susceptibility to a prototype non-viral (Toxoplasma) and viral (herpes simplex) intracellular pathogen at a cell and molecular biological level. It is based on the observation that macrophages (MO), lymphocytes and cytokines are important determinants of host defense to these pathogens and our finding that the major relevant deficit in neonatal monocyte-MO function is the decreased production of MO activating factor(s), primarily interferon-Gamma (IFN-Gamma) by neonatal lymphocytes. We will examine the hypothesis that decreased production of IFN-Gamma is due to decreased transcription of specific mRNA in spite of normal transcription and expression of IL2 and IL2 receptor in neonatal T cells. This will be determined using Northern blot, in situ RNA hybridization and nuclear run-off transcription assays. Decreased IFN-Gamma mRNA transcription is hypothesized to be developmentally (age) regulated and due in part to intrinsic differences in the state of the IFN-Gamma gene and in part to differences in T cell membrane transduction of mitogen/antigen and monocyte/MO signals for IFN-Gamma gene transcription. The molecular basis for the enhanced microbicidal activity of IFN-Gamma activated MO will also be explored. The dependence of oxygen-dependent and oxygen-independent mechanisms on new RNA and protein synthesis and on protein kinase C activity will be determined. The role of newly synthesized proteins, particularly in oxygen independent mechanisms, will be examined focusing initially on tumor necrosis factor. Studies of neonatal MO and lymphocyte defenses against herpes simplex will explore causes of decreased virostatic and cytolytic activity of these cells. Decreased release of the soluble static and lytic mediators lymphotoxin and tumor necrosis factor are hypothesized. These studies will be done with cells from normal infants and adults and longitudinally with cells from herpes simplex-infected infants, mothers and fathers. These studies will help characterize mechanisms of host defense against these pathogens and the basis for the neonates' decreased defenses. Such studies come at a time when immunological intervention in the treatment of such infections may soon be feasible and should serve to focus subsequent efforts at immunological intervention.